1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or facsimile apparatus which forms an image by an electrophotographic method, and a control method thereof.
2. Description of the Related Art
These days, image forming apparatuses using the electrophotographic method are achieving higher speeds and higher qualities. In particular, color image forming apparatuses require accurate color reproduction and tint stability, and generally have a function of automatically controlling the image density.
In image density calibration control, an image density detector incorporated in an image forming apparatus detects a plurality of test toner images (patches) which are formed on an image carrier while changing image forming conditions. The detected toner images are converted into a substantial amount of toner adhesion, and optimum image forming conditions are determined based on the conversion result.
A plurality of types of image density calibration control operations is generally executed to obtain optimum values for a plurality of types of image forming conditions. The types of image forming conditions include conditions such as the charging voltage, exposure intensity, and developing voltage, and a lookup table setting used to convert a signal input from the host into output image data when forming a halftone image. The tint varies depending on a change of the environment where an image forming apparatus is used, the use log of various consumables, and the like. The image density calibration control needs to be periodically executed to always stabilize the tint.
According to the detection principle of an optical image density detector, a light receiving element receives light which is emitted from a light emitting element and reflected by a patch or image carrier itself. The amount of toner adhered to the patch is calculated from the received light. Conversion into a substantial amount of toner adhesion is executed based on the relationship between an output from the light receiving element when no toner is adhered to the image carrier, and an output from the light receiving element when toner is adhered to the image carrier.
The reflectance of the image carrier surface changes depending on the position of the image carrier. To calculate the amount of toner adhesion at high precision, outputs in the presence and absence of toner need to be acquired at the same position on the image carrier. In general, a background output VB from the light receiving element in the absence of toner is acquired at a specific position. Then, the image carrier rotates at least one round. A patch is formed at the same position to acquire a patch output VP from the light receiving element. The background output VB corresponds to light reflected by the background of the image carrier. The patch output VP corresponds to light reflected by the patch. Specifying the position on the image carrier requires the circumference of the image carrier. This is because the time taken for a specific position on the image carrier to rotate is obtained by dividing the circumference by the circumferential speed (process speed) of the image carrier.
However, the circumference of the image carrier changes depending on variations of components, the environment of the image forming apparatus, and the like. If the circumference is used as a fixed value, an error occurs in specifying a position. To prevent this, information on the circumference of the image carrier needs to be measured dynamically.
Japanese Patent Laid-Open No. 10-288880 proposes the following method for an image forming apparatus which employs an intermediate transfer method. More specifically, a mark is attached to the surface of an intermediate transfer member. An optical sensor receives light reflected by the mark to measure the circumference of an image carrier. The mark is attached not to an image-formed surface used for image formation, but to a longitudinal end on the intermediate transfer member.
Japanese Patent Laid-Open No. 2006-150627 proposes a method of measuring the circumference of an electrostatic attraction belt in an image forming apparatus which adopts a direct transfer method. More specifically, according to the method disclosed in Japanese Patent Laid-Open No. 2006-150627, a patch is formed immediately below an optical image density detector. The optical image density detector measures the circumference of a target electrostatic attraction belt.
However, the conventional techniques suffer the following problems. For example, in the image forming apparatus in Japanese Patent Laid-Open No. 10-288880 that adopts the intermediate transfer method, the intermediate transfer member needs to rotate up to the mark set position, and further rotate one round. This is because when measurement of the circumference starts, the mark is not always positioned near the optical sensor. In the worst case, no circumference can be detected unless the intermediate transfer member rotates almost two rounds. A long circumference measurement time prolongs the period (so-called downtime) during which no image can be formed, impairing usability.
Even if usability can be maintained, the cost rises owing to an optical detection mark and optical sensor for measuring the circumference of an intermediate transfer member, as described above.
The image forming apparatus disclosed in Japanese Patent Laid-Open No. 2006-150627 forms a circumference measurement patch, consuming a larger amount of toner, compared to a case wherein no patch is formed. For the user, it is desirable to save toner as much as possible. In some cases, cleaning may take a long time.
Further, for example, immediately after activation upon return from a jam, the image carrier may travel unstably. In this case, the positional relationship between the circumference detection mark and the light receiving element changes, and the received light quantity varies between rounds. The image carrier circumference measurement method in Japanese Patent Laid-Open No. 10-288880 cannot obtain an accurate quantity of received light out of reflected light until traveling of the belt stabilizes. As a result, erroneous circumference information may be detected. Owing to even another factor, erroneous circumference information of the image carrier may be detected. If image density calibration control or the like is done based on the erroneous circumference information of the image carrier, no accurate image density calibration control result can be attained.